Extreme ultraviolet lithography is a promising technology for resolving integrated circuit (IC) feature sizes of 50 nm and below. There are many problems in order to realize EUV lithography and the most serious problem is to develop the EUV radiation source. An EUV source with a collectable radiation power of 50 W to 150 W at over 5 kHz in the spectral range of 13-14 nm will be required to achieve requirements for high volume manufacturing of 300 mm wafers.
Electrical discharge gas plasma devices (EUV lamps) are under investigation as promising EUV sources. The principle consists of heating up certain materials into plasma to such a level that the material emits EUV radiation. Potential source materials that emit EUV radiation at excited energy levels include xenon, oxygen, and lithium. The aim is to produce as many photons as possible in the required wavelength range. A pulsed discharge of electrically stored energy across a gap between an outer electrode and a central electrode is used in the presence of the gas for the creation of plasma with temperatures of several 100,000 C. This plasma emits radiation in the spectral range of around 10 nm to 20 nm.
FIG. 1 is a cross-sectional view of a basic representation of an electrical discharge gas plasma head 10 capable of producing EUV-emitting plasma 20. The plasma head 10 comprises an electrically conductive annular outer electrode 12 electrically insulated from a centrally positioned central electrode 14 by an insulator base 16 or ring separator. Gas 22 is expelled out of an orifice 15 at the tip 16 of the central electrode 14 as an electric discharge 17 is produced across the gap 18 between the outer electrode 12 and central electrode 14 thereby creating plasma 20. The gas 22 is energized by the electric discharge 17 forming ionized hot plasma 20. The plasma 20 is compressed, or pinched, due to electromagnetic forces of the electrical current through the discharge head 10 producing a highly dense source that emits EUV radiation 22.
In operation, a tremendous heat load, on the order of 5 kW/cm2, is experienced by the components of the plasma head 10. The components are only a few millimeters from the plasma 20, and in an erosive environment that quickly damages the components. This erosion severely effects performance, lifetime and reliability of the discharge head 10.
The outer electrode 12 and central electrode 14 are commonly made from refractory metals, such as tungsten or molybdenum which are more resistant to the effects of extreme heat. These materials are expensive, difficult to machine, and are prone to cracking when structurally loaded under severe heating conditions. These materials, none the less, erode over time in this environment.
The components of the electrical discharge plasma head 10 are subjected to extreme heating followed by rapid cooling. In particular, this temperature environment causes extreme thermal stresses and fracture of the insulator material rendering it an ineffective insulator. Furthermore, during the rundown and pinch phases of pulsed plasma operation, damaging amounts of photon, electron, and ion radiation bombards the components. This causes erosion of the insulator material into the plasma, which has a deleterious effect on plasma formation, stability, and output power.
The common electrode insulator materials are ceramics, which include alumina and sapphire, but are prone to thermal cracking and erosion in these environments. Alternate materials, such as Nitroxyceram and IRBAS, avoid catastrophic failure but have high erosion rates as heat loads are increased.
In order for the electric discharge plasma EUV sources to meet commercial requirements and demands, including reliability and productivity, lifetime-extending improvements will have to be made for the insulator components of the electric discharge gas plasma head.